I am finishing a walkout basement, and realized one window jamb is out of plumb by about 1/2": the bottom sticks out to be flush with sheetrock inside, but the top is flush with the framing. The top tips toward the outside.

The wall is plumb on the inside but not on the outside. I suspect the builder furred out the wall so it is thicker at the top than the bottom. (The house is a modular, so the basement walls had to be built first, then adjusted to match the upper house.) If so, and the window was put in flush to the outside framing, that would explain the tilt. The wall is 2x6, so there is likely an extension on the jamb.

The window is a new Anderson 200 double hung, and I am guessing it is installed with a flange, so the siding would have to be removed to get the window out. If so, I would rather not do that now, since it I can take out the window some other day if I choose so.

Questions:
(1)am I correct that the window can be removed from the outside if the siding is removed?
(2) is there a slick way to trim the inside of the window, such as buy another trim extension, cut it, and replace the existing one?

Sorry, its hard to explain in text.

Yes,if the window is a "new construction"style with the nailing flange on the outside(under the siding)you could adjust it by removal,cutting the flange,and repositioning,not the best way to go,if your walls are furred(as you suspect)it would just change one problem for another.

To the extension,yes also,removing the old piece and replacing it with new trim exactly the size of the opening would be the easier way(in my opinion)

Hope this helps,
Ken

PWd77:: kbc has give you very good advise as how to take care of your window problem:::: good advise KEN ;;;;; F. B.E.